PLANTING AND CARE OF THE ORCHARD. 41 



(c) The cultivation of the soil also sets plant food free. It opens 

 the soil up so that the air can get in and assist in the devel- 

 opment of the plant food locked up in an insoluble form. This 

 is an important function of tillag-e. The soil should be regarded 

 as a great laboratory in which many complex organic and inor- 

 ganic changes are continually taking place. 



Humus is the decayed organic matter found in soils. It 

 much resembles common charcoal in its physical properties, has 

 great affinity for water and holds fertilizing agents in the soil. 

 Humus makes the soil porous so that the air can readily pene- 

 trate it and also assists in chemical changes. Since it keeps 

 the soil porous it prevents it from baking hard, thus protecting 

 it from drought. It is largely on account of these qualities of 

 humus that stable manure, a large per cent of which is humus, 

 is often so much more effective than commercial fertilizers. The 

 plowing in of green crops adds humus to the soil. The contin- 

 ued cultivation of the soil sometimes affects it injuriously by 

 oxidizing all the humus and leaving it in such a condition that 

 it washes badly and will not hold moisture. A good illustration 

 of this change is shown in the fact that newly cleared land will 

 seldom wash badly until the decayed organic matter in the soil 

 has become so thoroughly oxidized that the humus no longer holds 

 the soil together. After this organic matter has disappeared clay 

 soils lose their loose, porous texture and become hard, com- 

 pact and liable to wash. 



The value of humus in the soil is well illustrated by experi- 

 ments at the Minnesota Experiment Station. Corn, grown con- 

 tinuously for six years on the same field, yielded an average of 

 21.4 bushels per acre, while corn in a three-rotation, including 

 wheat and clover, yielded 47.1 bushels per acre. As the plot 

 producing corn continuously has sufficient plant food to mature 

 a normal crop annually, the low yield can only be accounted for 

 by the poor physical condition of the soil produced through the 

 depletion of the humus by frequent cultivation. 



Growing fruit trees in sod is a practice not to be generally 

 recommended, but where the soil conditions are especially favor- 

 able it may sometimes be done to good advautage. In locations 

 wh9re trees are especially liable to injury from drought it is a 



